If it's solely solely solely for a RTT, then I'd consider just the 3-piece Nissan Roof Cross bars that (genuine Nissan) can be had new on fleabay for around $350 all up. They're easy to take one an off - in fact it may be easier to permanently lock it to the RTT and then unbolt the racks, rather than the RTT each time you take it off - I've seen that done before...
Would love to see your Fouby Fitouts all done up. I'm trying to find a place that will put in a permanent draw system just on the right hand side - so I still have 7 seats when I need them (and I do).